Flooded lead acid - how to calculate a practical AH rating?

[KevinJones]

I have 2 banks of Crown CR-390 batteries (L16 type). The 20 hour rating is 390 AH. My understanding is that is an absolute all-the-battery-can-give rating, and running a battery that way would basically bring the battery to dead. Eg, if I pulled 19.5A from each battery for 20 hours, they would be flat dead.

Of course I don't want to ever let them get that low; I wouldn't want them to go below something like 11.5V.

Is there a way to calculate, or at least a rule-of-thumb to come up with a 20 hour rating that would not let them get below 11.5V? Like say, for example, maybe 1/2 the rated AH for practical use, or something like that?

 

[timselectric]

Yes
50% DOD (depth of discharge), is the general rule for LA (Lead Acid) batteries.

 

[sunshine_eggo]

I have 2 banks of Crown CR-390 batteries (L16 type). The 20 hour rating is 390 AH. My understanding is that is an absolute all-the-battery-can-give rating,

Nope. It's the Ah produced when you apply a 390/20 = 19.5A load

That battery also has a C100 rating, see the datasheet:

https://cdn.shopify.com/s/files/1/1579/5179/files/CR-390_PowerSheet_D06390_0213.pdf?16021135676618962151

If you only pull 3.9A, it will deliver 435Ah.

If you pull 390/5 = 78A, you will only get 310Ah.

There are also the discharge capacity minutes for reference. As you can see a 25A load will last LONGER than 3X with a 75A (3x higher current) load.

Peukert's law - Wikipedia

en.wikipedia.org

and running a battery that way would basically bring the battery to dead. Eg, if I pulled 19.5A from each battery for 20 hours, they would be flat dead.

Mostly.

Of course I don't want to ever let them get that low; I wouldn't want them to go below something like 11.5V.

Depends.

Is there a way to calculate, or at least a rule-of-thumb to come up with a 20 hour rating that would not let them get below 11.5V? Like say, for example, maybe 1/2 the rated AH for practical use, or something like that?

No, because you are likely drawing a variable load all throughout the discharge, and a 20A load when the battery is at 60% will have a different voltage than when under a 50A load.

And you shouldn't just let them sit around at lower voltage just because they're not past some threshold. If you can charge the batteries, they should be fully charged and floated whenever possible.

Spend $117 on a Victron smartshunt. It actually tracks the state of charge of the battery. It can be programmed with a Peukert factor that allows it to maintain an accurate SoC based on the actual C rate of the battery. Additionally, with the temperature sensor (purchased separately), it can factor temperature into the battery efficiency to make SoC even more accurate.

The datasheet indicates 1200 cycle life with 50% DoD. 50% is the rule-of-thumb for lead acid batteries.

If these are subject to temperature fluctuations, you should be charging with temperature compenstation.

Lastly, these are flooded lead acid. You need to be neurotic about maintenance. Very easy to kill flooded lead acid with neglect.

 

[KevinJones]

Thank you both for the helpful answers.

 

[KevinJones]

And you shouldn't just let them sit around at lower voltage just because they're not past some threshold. If you can charge the batteries, they should be fully charged and floated whenever possible.

...

Lastly, these are flooded lead acid. You need to be neurotic about maintenance. Very easy to kill flooded lead acid with neglect.

The system is strictly for power outage backup. So most of the time there is no drain on the batteries, and they are on float charge full time from solar and are at about 13.5V during the day. They get a bulk charge every morning at 14.5V. Once a month they are equalized to 15.5V for two hours. After the EQ cycle is when I top them off with water. They typically take 1/2 to 3/4 gallons of water for 2 banks x 12 batteries. None of the cells have never been low on water, or anywhere close.

Power outages occur on an average of about once every 2 - 3 months, with duration ranging from 1 to 3 hours, most being about 1 hour. We had one recently which lasted 10 hours, but that was the only time in the ~10 years they've been in service. Most of them occur at night, usually during a storm or severe weather.

I was trying to get a ballpark idea of how much power could be used on one of the systems (the other strictly runs a well pump) for 10 hours without pulling the batteries too low. Currently only critical devices are powered, but wondered how much I can go beyond that.

 

[KevinJones]

I also want to mention that the batteries are connected using copper bus bars* rather than cables. The parallel bus bars are a single continuous bar (6 holes) and the + and - bars are identical. The series-connecting bars are identical as well. The holes were drilled at equal distances (precisely as possible) before bending. This was to ensure as much as possible equal distribution electrically. This seemed much better to me than using a bunch of short cables to connect everything up as I have seen done.

Each bank is 2-series x 6-parallel batteries for a 12V system. Yes, I know this is more than the recommended 3-parallel maximum, but hopefully the bus bar setup is helping mitigate inequalities.

Maybe I will get a pic when I am down there and post it.

*Bus bars are 1/8" x 1" IIRC; I believe what I used was nearly the same conductance/ft as 4/0 cable.

 

[sunshine_eggo]

The system is strictly for power outage backup. So most of the time there is no drain on the batteries, and they are on float charge full time from solar and are at about 13.5V during the day. They get a bulk charge every morning at 14.5V. Once a month they are equalized to 15.5V for two hours.

This may or may not be a good idea. Equalization charging is stressful. When needed, the benefit outweighs the negatives. I would not recommend an arbitrary EQ cycle for a standby application.

After the EQ cycle is when I top them off with water.

Which is when you shouldn't do it UNLESS you topped them off a few days ago, and they lost electrolyte from the EQ cycle. You should top them off a few days prior to an equalization charge AFTER they have been in float for a couple hours. A couple days later, conduct SG checks according to your schedule and EQ if needed.

They typically take 1/2 to 3/4 gallons of water for 2 banks x 12 batteries. None of the cells have never been low on water, or anywhere close.

That's a very high consumption rate, and it's likely due to unnecessary EQ cycles.

Is there a reason you're not simply floating them full on on an AC charger? Daily absorption charges without overnight usage is yet another unnecessary stressor to the batteries.

Standby batteries should be treated as standby batteries - keep them floated until you need them. If you only need them every 2-3 months, wait to run an EQ cycle after a usage event.

 

[KevinJones]

This may or may not be a good idea. Equalization charging is stressful. When needed, the benefit outweighs the negatives. I would not recommend an arbitrary EQ cycle for a standby application.

....

Is there a reason you're not simply floating them full on on an AC charger? Daily absorption charges without overnight usage is yet another unnecessary stressor to the batteries.

The monthly EQ cycle was recommended by a Crown engineer, after I told him my setup when I first purchased the batteries. He was aware that this was strictly a standby setup. I gave him the same details as I have given here. As I was told to do this from a Crown engineer, I'd like to hear another opinion on this.

I simply chose to charge them using solar to save wear and tear on the inverters, as well as keep the noise level down (the Trace inverters make quite a buzz when not on silent mode). But this practice can be changed.

 

[time2roll]

For emergency use I would run them down about 80% if no other power source is available. If there is a charging power source available I would try not to go below 40% charged. Less discharge is always better. Should get at least 400 cycles out of an L16 so 40 cycles per year gives 10 year service.

Yes I believe the longevity will be increased by cycling them every 3 to 8 weeks. Go down about 10% to 20% then charge and equalize.

 

[KevinJones]

I wrote Crown about the EQ recommendation and the suggestions I got here about not overdoing the EQ. I gave them a detailed description of my setup and use case as I have done here. This is the response I got:

From Crown Batteries:

With the string essentially on standby power and barely in use, I could see a case to where a monthly equalization may cause strain on the batteries. As long as you are achieving Specific Gravity readings of 1.265 you can afford to back off slightly but keep in mind that an equalization charge not only purges residual acid from the plates back into the electrolyte (water & sulfuric acid mix), but it also re-mixes the electrolyte. This prevents the acid from concentrating on the bottom half of the battery thus stratifying the plate. With every system being unique and being used in different capacities, it is difficult telling you what a perfect solution would be. This is why we recommend an equalization charge every 4-6 weeks (if you are not going below 50% Depth of Discharge), to maintain a balance of not causing undue strain to the plates but also staving off sulfation for as long as possible.

So, yes, backing off to a 6 week eq. charge should be fine, but I would not push it beyond much further. In any scenario, be sure to confirm you are achieving 100% SOC via SG readings when making adjustments.

 

[sunshine_eggo]

I wrote Crown about the EQ recommendation and the suggestions I got here about not overdoing the EQ. I gave them a detailed description of my setup and use case as I have done here. This is the response I got:

From Crown Batteries:

With the string essentially on standby power and barely in use, I could see a case to where a monthly equalization may cause strain on the batteries. As long as you are achieving Specific Gravity readings of 1.265 you can afford to back off slightly but keep in mind that an equalization charge not only purges residual acid from the plates back into the electrolyte (water & sulfuric acid mix), but it also re-mixes the electrolyte. This prevents the acid from concentrating on the bottom half of the battery thus stratifying the plate. With every system being unique and being used in different capacities, it is difficult telling you what a perfect solution would be. This is why we recommend an equalization charge every 4-6 weeks (if you are not going below 50% Depth of Discharge), to maintain a balance of not causing undue strain to the plates but also staving off sulfation for as long as possible.

I like that answer since it's coupled with a SG criterion. Most of the other manufacturers have SG criteria for triggering EQ rather than an arbitrary calendar EQ.

So, yes, backing off to a 6 week eq. charge should be fine, but I would not push it beyond much further. In any scenario, be sure to confirm you are achieving 100% SOC via SG readings when making adjustments.

This can be accomplished by ensuring you have been in float for at least two hours following a full bulk/absorption cycle.

 

[KevinJones]

...This can be accomplished by ensuring you have been in float for at least two hours following a full bulk/absorption cycle.

Are you saying this is in lieu of SG testing? Or am I not understanding your comment?

 

[sunshine_eggo]

Are you saying this is in lieu of SG testing? Or am I not understanding your comment?

That's when you take your SG reading.

At least two hours after float
draw in and expel electrolyte 3 times before taking a reading.

 

[KevinJones]

That's when you take your SG reading.

At least two hours after float
draw in and expel electrolyte 3 times before taking a reading.

Okay, thanks for the clarification.

 

[AI6MK]

I have 2 banks of Crown CR-390 batteries (L16 type). The 20 hour rating is 390 AH. My understanding is that is an absolute all-the-battery-can-give rating, and running a battery that way would basically bring the battery to dead. Eg, if I pulled 19.5A from each battery for 20 hours, they would be flat dead.

Of course I don't want to ever let them get that low; I wouldn't want them to go below something like 11.5V.

Is there a way to calculate, or at least a rule-of-thumb to come up with a 20 hour rating that would not let them get below 11.5V? Like say, for example, maybe 1/2 the rated AH for practical use, or something like that?

The C20 and C100 rating for a battery is to allow you to compare different batteries. As has been noted in the thread, for Lead/Acid chemistry you don't really want to discharge the battery below 50% or you will degrade the cycles*. For other chemistries, like LiFePO4 you can achieve 20% or lower SOC without affecting the lifetime.

*The CR390 datasheet says that if you discharge from 100% to 0% you can expect to get 500 cycles and from 100% to 50% you can expect 1200 cycles.

PS. I am a little surprised that you are using Lead/Acid technology when there are so many LiION avaliable at a reasonable price, not to mention the weight advantage. For my first solar setup I used 2x Trojan T105's and they were very good, but I then switched to used LEAF NMC batteries and now to LiFEPO4. I would never go back to using Lead/Acid.

 

[KevinJones]

That's when you take your SG reading.

At least two hours after float
draw in and expel electrolyte 3 times before taking a reading.

Is taking SG readings once a month sufficient in my case?

Also, do I need to check every single cell in every single battery, every time? That's 72 readings :-/

 

[sunshine_eggo]

Is taking SG readings once a month sufficient in my case?

Also, do I need to check every single cell in every single battery, every time? That's 72 readings :-/

Welcome to proper FLA maintenance. It sucks.

Monthly should be sufficient. It should be done over two days:

Day 1: check all cells for fluid level when fully charged and top off all cells.
Day 2: after being in float for at least two hours, check all 72 cells, 3 draw-in and expel before taking readings.

Once you've established that all 72 cells are working together for a few months, you can switch to a rolling single cell check:

Months 1-3: check all cells.
Month 4: check cell #1 in all batteries.
Month 5: check cell #2 in all batteries.
Month 6: check cell #3 in all batteries.
Month 7: check all 72
Month 8: goto month 4.

If you detect any outliers in any individual battery, check all cells in that battery.

If you're unable to attain SG 1.265 or if cells within a battery deviate by more the 0.03, an EQ charge is warranted.

 

[KevinJones]

I've finally gotten around to posting the results of my SG tests.

Note that my SG tester only has defined markings at 1.250, 1.265, 1.275 and 1.300.

I checked bank #1, and all readings were pretty much square between 1.275 and 1.300 (1.287ish?). There were a couple that were at appox. 1.280 (looking like the reading in the pic).

I then checked bank #2, and started finding cells a little below 1.265 after checking 2 batteries. So I then did an EQ charge on that bank, after topping off with water.

I then checked bank #2 again. This time all cells were between ~1.265 and ~1.285, with most being ~1.280 (like the reading in the pic). That is with the exception of 3 cells. 2 cells read slightly below 1.265 (~1.263 - 1.263), and 1 cell read just above the 1.250 mark :-/

No battery had cells deviating > .03.

Note that the previous EQ charge was only about 7 weeks before this last one on bank #2. And IIRC, we had not sustained power outages during that time.

Feedback would be much appreciated.

 

[KevinJones]

Oops, forgot to post the pic of the SG tester:

 

[KevinJones]

And here is the pic of the bus bar system I said I would post:

(Note, this is much easier to do with Trojan batteries where you can use a single straight strip of bus bar for all parallel connections. I don't know why Crown didn't think of this.)